For electronics, microelectronics and micro-electromechanics, semiconductor wafers with extreme requirements for global and local planarity, one-side referenced local planarity (nanotopography), roughness and cleanness are needed as starting materials (substrates). Semiconductor wafers are wafers of semiconductor materials, in particular compound semiconductors such as gallium arsenide and predominantly elementary semiconductors such as silicon and sometimes germanium. According to the prior art, semiconductor wafers are produced in a multiplicity of successive process steps: in a first step, for example, a single crystal (rod) of semiconductor material is pulled by the Czochralski method or a polycrystalline block of semiconductor material is cast, and in a further step the resulting circular-cylindrical or block-shaped workpiece of semiconductor material (ingot) is cut into individual semiconductor wafers by wire sawing.
Wire saws are used in order to cut a multiplicity of wafers from a workpiece made of semiconductor material. U.S. Pat. No. 5,771,876 describes the functional principle of a wire saw, which is suitable for the production of semiconductor wafers. The components of these wire saws include a machine frame, a forward feed device and a sawing tool, which consists of a web (wire web) of parallel wire sections.
In general, the wire web is formed by a multiplicity of parallel wire sections which are tensioned between at least two wire guide rollers, the wire guide rollers being rotatably mounted and at least one of them being a driven roller.
The wire sections may belong to a single finite wire, which is guided spirally around the roll system and is unwound from a stock roll onto a receiver roll. Patent specification U.S. Pat. No. 4,655,191, on the other hand, describes a wire saw in which a multiplicity of finite wires are provided and each wire section of the wire web is assigned to one of these wires. EP 522 542 A1 also describes a wire saw in which a multiplicity of endless wire loops run around the roll system.
The sawing wire may be covered with a cutting layer. When using wire saws having a sawing wire without firmly bound abrasive, abrasive in the form of a suspension (cutting suspension, sawing slurry, slurry) is supplied during the cutting process. During the cutting process, the workpiece passes through the wire web, in which the sawing wire is arranged in the form of wire sections lying parallel to one another. The passage through the wire web is brought about by means of a forward feed device, which moves the workpiece against the wire web, the wire web against the workpiece or the workpiece and the wire web against one another.
When cutting semiconductor wafers from a workpiece made of semiconductor material, it is conventional for the workpiece to be connected to a sawing strip into which the sawing wire cuts at the end of the process. The sawing strip is for example a graphite strip, which is adhesively bonded or cemented on the lateral face of the workpiece. The workpiece with the sawing strip is then cemented on a support body. After the cutting, the resulting semiconductor wafers remain fixed on the sawing strip like the teeth of a comb, and can thus be removed from the wire saw. Subsequently, the remaining sawing strip is separated from the semiconductor wafers.
The production of semiconductor wafers from workpieces made of semiconductor material, for example from circular-cylindrical single crystal rods or cuboid polycrystalline blocks, places great demands on the wire sawing. The aim of the sawing process is generally for each sawed semiconductor wafer to have side faces which are as plane as possible and lie parallel to one another. The so-called warp of the wafers is a known measure of the deviation of the actual wafer shape from the desired ideal shape. The warp should generally amount to at most a few micrometers (μm). It results from a relative movement of the sawing wire sections relative to the workpiece, which takes place in the axial direction with respect to the workpiece in the course of the sawing process. This relative movement may for example be caused by cutting forces which occurred during the sawing, axial displacements of the wire guide rolls due to thermal expansion, bearing plays or thermal expansion of the workpiece.
A significant amount of heat is released when the workpiece is cut by the abrasive, which in the course of the sawing process leads to heating of the workpiece and therefore to thermal expansion. This in turn leads not only to an increase of the warp, but also to significant waviness of the sawed wafers. A particularly strong temperature increase takes place over the first millimeters of the cut after cutting into the workpiece. With an increasing engagement length, the temperature of the workpiece increases further. In the region of the maximum engagement length, the workpiece temperature also reaches its maximum and subsequently decreases slightly, which besides the decreasing cutting heat is also attributable to the cooling fin effect of the resulting wafers. Workpiece temperature changes of +/−5° C. during the wire sawing have a negligible effect on the warp and the waviness, but cannot generally be achieved without additional outlay.
Since the workpiece is heated during the wire sawing process in order to produce semiconductor wafers, it needs to be cooled continuously during the sawing process in order to prevent thermally induced expansion of the workpiece and therefore to prevent a perturbing curvature of the cutting profile. Various methods for cooling the workpiece during wire sawing are known. In EP 2 070 653 A1, the cooling rate of the workpiece is monitored and an additional coolant (cooling slurry, slurry) is added when the cutting depth of the sawing wire is ⅔ or more of the diameter of the workpiece. EP 1097782 B1 and DE 10122628 A1 describe methods in which a thermally regulated coolant is applied onto the workpiece.
The disadvantage of the aforementioned methods is that the coolant flows into the region of the cut, where it is mixed with the cutting suspension. The choice of a suitable coolant is therefore greatly restricted, and it must have a similar composition to the cutting suspension. Even when using exactly the same media for the coolant and the cutting suspension, the problem remains that the coolant detrimentally affects the cutting behavior.
The detrimental effect is due to the fact that the temperature of the coolant for optimal cooling of the crystal must be at a different level from the temperature of the cutting suspension. However, the temperature of the cutting suspension is crucial for its viscosity, which determines the transport properties for the abrasive and therefore in turn affects the quality of the cut. The temperature of the cutting suspension is therefore usually regulated accurately, and optionally varied in a controlled way during the cutting. Particularly when using glycols as a carrier medium in the cutting suspension, there is a large temperature dependency of the viscosity.
Furthermore, the coolant flowing onto the cutting web also affects the temperature of the cutting suspension. This leads to an undesirable change of the wire web temperature, which results in a degradation of the quality of the cut.
US 2010/163010 A1 describes a method in which an attempt is made to avoid this problem. In this case, the coolant is alternately switched off and on so that the coolant is only applied onto the side of the workpiece on which the sawing wire leaves the workpiece. This is intended to prevent the coolant from being mixed directly with the sawing suspension which enters the sawing kerf. However, coolant still reaches the wire web and—albeit to a reduced extent—alters the thermal and mechanical conditions of the cut. In this method, it is necessary to use exactly the same medium for the coolant and the cutting suspension, since otherwise the cutting suspension would be uncontrollably modified.
JP 2005 329506 A2 describes a method in which a cooling gas is blown as a coolant onto the sawing wire. In this way, the viscosity of the cutting suspension can likewise be impaired and the quality of the cut degraded. Furthermore, gases are only very limitedly suitable for cooling since their relatively low heat capacity does not ensure sufficient dissipation of the amounts of heat usually produced during the cutting.